A Massachusetts company says its experimental vaccine against the coronavirus showed encouraging results in very early testing. Cambridge-based Moderna Inc. said Monday that its vaccine triggered immune responses in eight healthy, middle-aged volunteers. The experimental vaccine generated antibodies similar to those seen in people who have recovered from COVID-19 in study volunteers who were given either a low or medium dose. The study is being led by the U.S. National Institutes of Health. In the next phase, researchers will try to determine which dose is best for a definitive experiment that they aim to start in July.